### Q.31:- The first (ΔiH1) and the second (ΔiH) ionization enthalpies (in kJ mol–1) and the (ΔegH) electron gain enthalpy (in kJ mol–1) of a few elements are given below:

Elements | Δ_{i}H1 |
Δ_{i}H |
Δ_{eg}H |

I | 520 | 7300 | -60 |

II | 419 | 3051 | -48 |

III | 1681 | 3374 | -328 |

IV | 1008 | 1846 | -295 |

V | 2372 | 5251 | +48 |

VI | 738 | 1451 | -40 |

### Which of the above elements is likely to be :

### (a) the least reactive element.

(b) the most reactive metal.

(c) the most reactive non-metal.

(d) the least reactive non-metal.

(e) the metal which can form a stable binary halide of the formula MX_{2}, (X=halogen).

(f) the metal which can form a predominantly stable covalent halide of the formula MX (X=halogen)?

**Answer:-**

(a) The element V has highest first ionization enthalpy (∆ _{i}H_{1}) and positive electron gain enthalpy (∆_{eg}H) and hence it is the least reactive element. Since inert gases have positive∆_{eg}H, therefore, the element-V must be an inert gas. The values of ∆ _{i }H_{1}, ∆ _{i}H_{2} and ∆_{eg}Hmatch that of He.

(b) The element II which has the least first ionization enthalpy (∆ _{i }H_{1}) and a low negative electron gain enthalpy (∆_{eg}H) is the most reactive metal. The values of ∆ _{i }H_{1}, ∆ _{i}H_{2} and ∆_{eg}H match that of K (potassium).

(c) The element III which has high first ionization enthalpy (∆ _{i}H_{1}) and a very high negative electron gain enthalpy (∆_{eg}H) is the most reactive non-metal. The values of ∆ _{i }H_{1}, ∆ _{i}H_{2} and ∆_{eg}H match that of F (fluorine).

(d) The element IV has a high negative electron gain enthalpy (∆_{eg}H ) but not so high first ionization enthalpy (∆_{eg}H). Therefore, it is the least reactive non-metal. The values of ∆ _{i }H_{1}, ∆ _{i}H_{2} and ∆_{eg}H match that of I (Iodine).

(e) The element VI has low first ionization enthalpy (∆ _{i}H_{1}) but higher than that of alkali metals. Therefore, it appears that the element is an alkaline earth metal and hence will form binary halide of the formula MX_{2}(where X = halogen). The values of ∆ _{i }H_{1}, ∆ _{i}H_{2} and ∆_{eg}H match that of Mg (magnesium).

(f) The element I has low first ionization (∆ _{i}H_{1}) but a very high second ionization enthalpy (∆ _{i}H_{2}), therefore, it must be an alkali metal. Since the metal forms a predominarly stable covalent halide of the formula MX (X = halogen), therefore, the alkali metal must be least reactive. The values of ∆ _{i }H_{1}, ∆ _{i}H_{2} and ∆_{eg}H match that of Li (lithium).